Consolidate logic for parsing relations. (#12693)

Parse the `m.relates_to` event content field (which describes relations)
in a single place, this is used during:

* Event persistence.
* Validation of the Client-Server API.
* Fetching bundled aggregations.
* Processing of push rules.

Each of these separately implement the logic and each made slightly
different assumptions about what was valid. Some had minor / potential
bugs.

1 files changed, 12 insertions, 18 deletions

diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index 4a4b535bae..0951b9c71f 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -44,7 +44,7 @@ from synapse.api.errors import (
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, RoomVersions
 from synapse.api.urls import ConsentURIBuilder
 from synapse.event_auth import validate_event_for_room_version
-from synapse.events import EventBase
+from synapse.events import EventBase, relation_from_event
 from synapse.events.builder import EventBuilder
 from synapse.events.snapshot import EventContext
 from synapse.events.validator import EventValidator
@@ -1060,20 +1060,11 @@ class EventCreationHandler:
             SynapseError if the event is invalid.
         """
 
-        relation = event.content.get("m.relates_to")
+        relation = relation_from_event(event)
         if not relation:
             return
 
-        relation_type = relation.get("rel_type")
-        if not relation_type:
-            return
-
-        # Ensure the parent is real.
-        relates_to = relation.get("event_id")
-        if not relates_to:
-            return
-
-        parent_event = await self.store.get_event(relates_to, allow_none=True)
+        parent_event = await self.store.get_event(relation.parent_id, allow_none=True)
         if parent_event:
             # And in the same room.
             if parent_event.room_id != event.room_id:
@@ -1082,28 +1073,31 @@ class EventCreationHandler:
         else:
             # There must be some reason that the client knows the event exists,
             # see if there are existing relations. If so, assume everything is fine.
-            if not await self.store.event_is_target_of_relation(relates_to):
+            if not await self.store.event_is_target_of_relation(relation.parent_id):
                 # Otherwise, the client can't know about the parent event!
                 raise SynapseError(400, "Can't send relation to unknown event")
 
         # If this event is an annotation then we check that that the sender
         # can't annotate the same way twice (e.g. stops users from liking an
         # event multiple times).
-        if relation_type == RelationTypes.ANNOTATION:
-            aggregation_key = relation["key"]
+        if relation.rel_type == RelationTypes.ANNOTATION:
+            aggregation_key = relation.aggregation_key
+
+            if aggregation_key is None:
+                raise SynapseError(400, "Missing aggregation key")
 
             if len(aggregation_key) > 500:
                 raise SynapseError(400, "Aggregation key is too long")
 
             already_exists = await self.store.has_user_annotated_event(
-                relates_to, event.type, aggregation_key, event.sender
+                relation.parent_id, event.type, aggregation_key, event.sender
             )
             if already_exists:
                 raise SynapseError(400, "Can't send same reaction twice")
 
         # Don't attempt to start a thread if the parent event is a relation.
-        elif relation_type == RelationTypes.THREAD:
-            if await self.store.event_includes_relation(relates_to):
+        elif relation.rel_type == RelationTypes.THREAD:
+            if await self.store.event_includes_relation(relation.parent_id):
                 raise SynapseError(
                     400, "Cannot start threads from an event with a relation"
                 )